Travis: Fix OS-X virtualenv issue; Reduce coveralls.

Details:
- This change fixes the issue that Travis runs on OS-X fail with
  an incorrect python version referenced in the hashbang of
  the virtualenv command. It also fixes an incorrect python
  version that was used on OS-X.
- This change also reduces the coveralls runs to only be run
  in one combination (py27 on linux with latest package level).

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

.travis.yml

@@ -147,19 +147,35 @@ install:
     echo "PIP_CMD=$PIP_CMD"
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       if [[ "${PYTHON:0:1}" == "2" ]]; then
-        OSX_PYTHON_PKG=python;
+        OSX_PYTHON_PKG=python@2;
       else
-        OSX_PYTHON_PKG=python3;
+        OSX_PYTHON_PKG=python;
       fi;
       echo "OSX_PYTHON_PKG=$OSX_PYTHON_PKG";
     fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew update;
     fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
-      brew search python && brew ls --versions $OSX_PYTHON_PKG && brew upgrade $OSX_PYTHON_PKG || brew install $OSX_PYTHON_PKG;
+      brew ls --versions $OSX_PYTHON_PKG;
+      rc=$?;
+      if [[ $rc == 0 ]]; then
+        echo "Upgrading $OSX_PYTHON_PKG";
+        brew upgrade $OSX_PYTHON_PKG;
+      else
+        echo "Installing $OSX_PYTHON_PKG";
+        brew install $OSX_PYTHON_PKG;
+      fi;
+      echo "Unlinking $OSX_PYTHON_PKG";
+      brew unlink $OSX_PYTHON_PKG;
+      echo "Linking $OSX_PYTHON_PKG";
+      brew link --overwrite $OSX_PYTHON_PKG;
+      $PIP_CMD install --upgrade pip setuptools;
+      echo "Checking Python and Pip";
+      which $PYTHON_CMD && which $PIP_CMD;
     fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+      $PIP_CMD install virtualenv;
       virtualenv $HOME/venv -p $PYTHON_CMD && source $HOME/venv/bin/activate;
     fi
   - env |sort
@@ -195,4 +211,6 @@ script:
   - make test
 
 after_success:
-  - if [[ -z $_MANUAL_CI_RUN && $TRAVIS_PYTHON_VERSION == 2.7 ]]; then coveralls; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" && "$TRAVIS_PYTHON_VERSION" == "2.7" && "$PACKAGE_LEVEL" == "latest" && -z $_MANUAL_CI_RUN ]]; then
+      coveralls;
+    fi