[BEAM-1769] Travis should run all tox tests

.travis.yml

@@ -68,7 +68,6 @@ before_install:
  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then jdk_switcher use "$CUSTOM_JDK"; fi
  - export BEAM_SUREFIRE_ARGLINE="-Xmx512m"
  # Python SDK environment settings.
- - export TOX_ENV=py27
  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then export TOX_HOME=$HOME/Library/Python/2.7/bin; fi
  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then export TOX_HOME=$HOME/.local/bin; fi
 
@@ -80,7 +79,7 @@ install:
  - rm -rf "$HOME/.m2/repository/org/apache/beam"
 
 script:
- - if [ "$TEST_PYTHON" ]; then travis_retry $TOX_HOME/tox -e $TOX_ENV -c sdks/python/tox.ini; fi
+ - if [ "$TEST_PYTHON" ]; then travis_retry $TOX_HOME/tox -c sdks/python/tox.ini; fi
  - if [ ! "$TEST_PYTHON" ]; then travis_retry mvn --batch-mode --update-snapshots --no-snapshot-updates --threads 1C $MAVEN_OVERRIDE install && travis_retry bash -ex .travis/test_wordcount.sh; fi
 
 cache: