Use CMAKE_CURRENT_BINARY_DIR consistently for installs

[jdavidpeter]

abedra/libvault#100 Currently CMAKE_CURRENT_BINARY_DIR is mixed with CMAKE_BINARY_DIR in the CMakeLists.txt. This creates errors when libvault is not built directly, but as part of a larger project and is referred by add_subdirectory()

[abedra]

Good catch, thanks