Gemfile - optionally skip loading WebYast gems

needed by `rake assets:precompile` when building RPM package to avoid copying asset files from other packages (installed by BuildRequires dependencies).

Reported by OBS (see https://build.opensuse.org/request/show/187286) - `rubygem-webyast-time` contains files from `rubygem-webyast-services` which cause file conflicts.

(As a bonus the `unless ... else ...` has been changed to more readable `if ... else ...`.)