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
restore old pkg-config file
cmake install pkg-config
Merge pull request #16 from da2ce7/pkg_config

cmake install pkg-config
cppcheck: fix variableScope in opentxs-core