Relx upgrade to version 3.20.0

Add OTP 19.0 to travis

Avoid PLT rebuild when files deleted on new dialyzer

Handle empty PLT or app when using dialyzer

Escriptize based on configured apps only

Fixes for windows environment: 1) copy empty directories when cloning…

Used a nonexisting option instead of a correct one

Prior to this patch, the escriptize command flat out selected all
declared dependencies.

This patch instead looks at the app files and only includes the
dependencies of the top level app and the extra ones, avoiding to
package more apps than required.

This required a version bump on cth_readable as it mistakenly included
'syntax_lib' instead of 'syntax_tools' as a dependency.

lock file contains expected hash for pkg dependencies

Let DEBUG="" and QUIET="" mean disable the option

use logs rather than skimming github pages

Fix unicode rendering of deps tree (issue #1140)

Setting DEBUG/QUIET environment variable to the empty string now acts
the same as unsetting it. Unsetting is not always easy/possible.

ignore return value of `rebar_utils:reread_config` in shell provider

normalize include dirs to absolute paths during compilation

changed include files were not properly picked up by `erlc_compiler`
in cases where they were in directories relative to the application
source and not the current working dir of rebar3

fixes #1199

REBAR-1184 always recompile if `ERL_COMPILER_OPTIONS` env var is set

Compiler source paths in output are now relative

partially addresses #1184

add support for passing a sys_config to common test

Expand os ENV proxy support to bootstrap script

Support ENV config for proxy values

- The configured stuff in rebar3 takes precedence over the ENV
- The env is then chosen

Write rebar3.crashdump on errors

This uses the env variable as a global store for variables. It's not the
cleanest thing, but it sounded nicer than pdicts.

This reworks the version and hash printing in the lockfile to minimize
diff changes:

- the version is on its own line so that the locks are mostly the same
  aside from the last line
- the hashes are each printed on one line with the package name for
  simpler diffing too.

This required moving the reporting functions to rebar_base_compiler but
since this was already done for error_tuple, this seems to make sense.

Paths are also reformatted for warnings in erlc files.

Fix compile error case when cover compilation

This will allow us to eventually change the default type without it
looking ridiculous.

Bugfix for upgrading deps of umbrella apps

Fix minor typo in README

The previous iteration of the patch worked somewhat by accident. After
digging in and figuring out why the two dep sources are the way they
are, the patch is now properly working with a well-documented
explanatiion inline.