Add option to skip package syncing and updating.

Add option --no-update and relative test.
Move package manager update into install_deps function.
Add early return in install_deps if $NO_UPDATE is set.
Merge branch 'master' of https://github.com/postmodern/ruby-install into no_update_option
Add option to skip installing deps before building Ruby.

Add option --skip-install-deps and relative tests.
Return early from `install_deps` if SKIP_DEPS is set.
Merge branch 'master' of https://github.com/postmodern/ruby-install into skip_deps_option

Conflicts:
	share/ruby-install/ruby-install.sh
	test/parse_options_test.sh
Tab indentation.
Merge pull request #39 from stefanozanella/skip_deps_option

Add option to skip installation of build dependencies.