[Plugin: gitignore] Several fixes in the plugin

This pull request joins together these 4 fixes:

- Changing the API URL from http to https, as [has](https://github.com/robbyrussell/oh-my-zsh/pull/3106) [been](https://github.com/robbyrussell/oh-my-zsh/pull/3114) [asked](https://github.com/robbyrussell/oh-my-zsh/pull/3118) [for](https://github.com/robbyrussell/oh-my-zsh/pull/3144) [a](https://github.com/robbyrussell/oh-my-zsh/pull/3171) [lot](https://github.com/robbyrussell/oh-my-zsh/pull/3172) [of](https://github.com/robbyrussell/oh-my-zsh/pull/3173) [times](https://github.com/robbyrussell/oh-my-zsh/pull/3180) [already](https://github.com/robbyrussell/oh-my-zsh/pull/3205). The commit used is the one in #3105, provided by @gugu, for being the first to submit the fix.
- Fixing a misspelling in the `_gitignoreio_get_command_list` function. Fix provided by @ashaindlin in #3225.
- Using the follow-redirects flag (`-L`) in case the URL changes again. Fix provided by @wooparadog in #3203.
- Using the silent flag in `gi()` too, as was asked by @yachi in https://github.com/robbyrussell/oh-my-zsh/pull/3105#issuecomment-58313493. 

All in all, the plugin now uses curl in `follow-redirects` and `silent` mode **in both calls**. This should satisfy most (if not all) user requests made.
Thank you all involved.