Disable JS shell for Servo

Should cut down compile time a little bit.
Fix errors linking debug builds with Servo

SpiderMonkey defines 'jsid' to be a struct type in debug builds, which changes
the mangled names of some functions, producing a linker error.

This feature serves only to catch mistakes in the SpiderMonkey C++ code, so
for Servo we just disable it.
Merge pull request #5 from kmcallister/debug-build

Fix debug build