Move $HOME/tmux/bin before other travis paths

it seems travis includes tmux now (1.6), we want our tmux to have higher
precedence.

Author: tony

.travis.yml

@@ -36,7 +36,7 @@ before_script:
   - git checkout $TMUX_VERSION
   - sh autogen.sh
   - ./configure --prefix=$HOME/tmux && make && make install
-  - export PATH=$PATH:$HOME/tmux/bin
+  - export PATH=$HOME/tmux/bin:$PATH
   - cd ..
   - tmux -V
 script: coverage run --source=tmuxp setup.py test