travis: use pip2 instead of pip

.. since now mac osx image expects pip2 or pip3, and doesn't know pip:

0.01s$ pip install --user cpp-coveralls
/Users/travis/.travis/job_stages: line 57: pip: command not found

Ref: https://github.com/travis-ci/travis-ci/issues/8829

Closes https://github.com/curl/curl/pull/2133
diff --git a/.travis.yml b/.travis.yml
index 7e2be4a..d292455 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -91,7 +91,7 @@
           env: T=fuzzer
 
 install:
-  - pip install --user cpp-coveralls
+  - if [ "$T" = "coverage" ]; then pip2 install --user cpp-coveralls; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update > /dev/null; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew reinstall libtool > /dev/null; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew install rtmpdump libssh2 c-ares libmetalink libressl nghttp2 libmetalink; fi