Fix directory creation on Linux Travis

Travis complains that the /home/travis/bin folder already exists when creating
it. Use mkdir with the -p option.

ci/travis/travis_install.linux.sh

@@ -3,7 +3,7 @@
 # We can't use sudo, so we have to approximate the behaviour of setting the
 # default system compiler.
 
-mkdir ${HOME}/bin
+mkdir -p ${HOME}/bin
 
 ln -s /usr/bin/g++-4.8 ${HOME}/bin/c++
 ln -s /usr/bin/gcc-4.8 ${HOME}/bin/cc