fix pip on osx

tools/travis/travis_osx_before_install.sh

@@ -1,12 +1,12 @@
 #!/bin/bash
 set -ev
 
-brew update
 brew install check
 brew install userspace-rcu
 brew install valgrind
 brew install graphviz
+brew install python
 
-pip install --user sphinx
-pip install --user breathe
-pip install --user sphinx_rtd_theme
+pip2 install --user sphinx
+pip2 install --user breathe
+pip2 install --user sphinx_rtd_theme