Fix clear cache flag checking

[ErikGartner]

The flag BUILDPACK_CLEAR_CACHE was checked in an improper way causing problem when the flag exists but is empty.

The new checking method is also more consistent with other flags in the compile file.

[chdsbd]

Looks great. Do you mind updating these other two places with your new method?

meteor-buildpack-horse/bin/compile

Line 181 in 055446a

if [ -n "${BUILDPACK_PRELAUNCH_METEOR+1}" ]; then

meteor-buildpack-horse/bin/compile

Line 30 in 055446a

if [ -n "${BUILDPACK_VERBOSE+1}" ]; then

[ErikGartner]

Done :)

[chdsbd]

Thanks for the PR!