travis: install new CMake on OS X

Before, the attempt to install a new version of CMake with Homebrew
failed because there already was an installed CMake version.
Merge pull request #14 from toxeus/travisPath

travis: install new CMake on OS X
Delete unused fast_mutex.h
Merge pull request #3 from lclc/deleteFastMutex

Delete unused fast_mutex.h
travis: leave system OpenSSL unchanged on OSX build

now compiling using un-force-linked OpenSSL 1.0.1 from brew
we shouldn't be changing the system OpenSSL, as this can cause
other applications to stop working on OSX