GLFW app not tested on travis

[springmeyer]

The GLFW debug app might regress so we need to start building it on travis.

[mourner]

Nope, for some reason Travis fails to build GLFW (https://travis-ci.org/mapbox/geojson-vt-cpp/jobs/140836998):

$ make build/debug V=1
clang++-3.8  -I include -std=c++14 -Wall -Wextra -O3 debug/debug.cpp -o build/debug `.mason/mason cflags variant 1.1.0` `.mason/mason cflags geometry 0.7.0` `.mason/mason cflags geojson 0.1.2-cxx03abi` `.mason/mason static_libs geojson 0.1.2-cxx03abi` `.mason/mason cflags glfw 3.1.2` `.mason/mason ldflags glfw 3.1.2` `.mason/mason static_libs glfw 3.1.2`
Package xxf86vm was not found in the pkg-config search path.
Perhaps you should add the directory containing `xxf86vm.pc'
to the PKG_CONFIG_PATH environment variable
Package 'xxf86vm', required by 'GLFW', not found
Package xxf86vm was not found in the pkg-config search path.
Perhaps you should add the directory containing `xxf86vm.pc'
to the PKG_CONFIG_PATH environment variable
Package 'xxf86vm', required by 'GLFW', not found
debug/debug.cpp:1:10: fatal error: 'GLFW/glfw3.h' file not found
#include <GLFW/glfw3.h>
         ^
1 error generated.
make: *** [build/debug] Error 1