Fix #20: Incorrect generation of relative file names
Bump version to v0.7.5 for GNU ELPA
Don't hard-code envvars in ggtags-save-project-settings
Require Emacs 24

Declaring lexical-binding:t implies that the code requires Emacs 24 or higher.
Merge pull request #21 from purcell/patch-1

Require Emacs 24
Don't run ggtags-update-tags in ggtags-completion-table

for efficiency because the latter is used by
ggtags-highlight-tag-at-point. Lower ggtags-oversize-limit.