I tried all the solutions posted here, but was not lucky. I reinstalled mysql with homebrew a couple of times and still no luck. Then I came across a blog post with a solution.
I edited the mysql_config file in /usr/local/Cellar/mysql/5.6.12/bin and removed
the W-compiler options
-Wno-null-conversion and
-Wno-unused-private-field
for cflags and cxxflags.
This solved the problem with gem install mysql2 and bundle install
Reference: http://www.randomactsofsentience.com/2013/05/gem-install-mysql2-missing-mysqlh-on-os.html