travis: don't (re)install python

The new Mac OS X images apparently already have python2 and python3,
and `brew` considers asking to install something already installed
as a fatal error...

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.travis.yml b/.travis.yml
index 97720cf..cc4d0b26 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -19,10 +19,10 @@
 before_install:
   - HOMEBREW_NO_AUTO_UPDATE=1 brew install expat gettext
   - if test "x$BUILD" = xmeson; then
-      HOMEBREW_NO_AUTO_UPDATE=1 brew install python3 ninja;
+      HOMEBREW_NO_AUTO_UPDATE=1 brew install ninja;
     fi
   - if test "x$BUILD" = xscons; then
-      HOMEBREW_NO_AUTO_UPDATE=1 brew install python2 scons;
+      HOMEBREW_NO_AUTO_UPDATE=1 brew install scons;
     fi
 
   # Set PATH for homebrew pip3 installs