Linux-proofing build system

[levirak]

Make use of gulp.src's options feature to manually designate the base and prevent the need for manual path modification. Likewise change a reference to {foundrydata}/data to {foundrydata}/Data to appease case-sensitive file systems.

These changes let me run npm run build and the like from my Linux machine. I do not have a Windows system for testing against.

Fixes #1392