Add env variable to identify extra tests

Add an env var to easily identify travis runs that have differing
requirements.

Author: Hugo Osvaldo Barrera

.travis.yml

@@ -36,10 +36,12 @@ matrix:
       python: 3.6
       after_install:
         - echo click_repl >> requirements.txt
+      env: TOXENV=py EXTRAS=click_repl
     - # Run tests with incompatible test dependencies:
       python: 3.6
       after_install:
         - echo pyicu >> requirements.txt
+      env: TOXENV=py EXTRAS=pyicu
 
 addons:
   apt: