Update after chatter change.
Correct precedence of GAMMALIB_CALDB over CALDB (Bug #1094)

GAMMALIB_CALDB should take precedence over CALDB, yet so far the opposite was true. This has now been corrected for.
The GAMMALIB_CALDB environment variable takes precedence over the CALDB environment variable.

This was the initial intention, but the code did the opposite.