Drop R15 support

It can't even fetch packages from Hex anymore because of old SSL/TLS
libraries, and so it can't bootstrap anymore either. Plus R20 comes out
soon, and 5 major versions is quite enough.

.travis.yml

@@ -1,8 +1,6 @@
 language: erlang
 matrix:
   include:
-    - os: linux
-      otp_release: R15B03
     - os: linux
       otp_release: R16B03-1
     - os: linux

src/rebar_prv_common_test.erl

@@ -567,9 +567,6 @@ get_tests_from_specs(Specs) ->
             case ct_testspec:collect_tests_from_file(Specs,true) of
                 Tests when is_list(Tests) ->
                     {ok,[{S,ct_testspec:prepare_tests(R)} || {S,R} <- Tests]};
-                R when is_tuple(R), element(1,R)==testspec ->
-                    %% R15
-                    {ok,[{Specs,ct_testspec:prepare_tests(R)}]};
                 Error ->
                     Error
             end