Don't rely on an environment variable for Travis

The environment variable approach breaks other projects' Travis-CI runs
if they include other projects that depend on a different version of
PropEr.

Fixes: #106

.travis.yml

@@ -1,6 +1,7 @@
 language: erlang
 notifications:
   email: paul.joseph.davis@gmail.com
+script: make check
 otp_release:
    - 17.5
    - 17.4

rebar.config.script

@@ -16,7 +16,7 @@ Proper = [
 ConfigPath = filename:dirname(SCRIPT),
 DevMarker = filename:join([ConfigPath, ".jiffy.dev"]),
 
-case filelib:is_file(DevMarker) orelse os:getenv("TRAVIS") == "true" of
+case filelib:is_file(DevMarker) of
     true ->
         % Don't override existing dependencies
         Config0 = case lists:keyfind(deps, 1, CONFIG) of