Require stable version of geshi library

This definitely needs to be addressed. This blocked me from installing geshifilter via composer, as my project required at least stability "alpha" which dev-master does not satisfy. Furthermore, the project page for this module explicitly warns *against* using the latest dev version of the geshi library and instructs using the stable 1.0.x branch.

For now, my workaround for using composer was to manually add geshi library to my composer.json like this:

"geshi/geshi": "1.0.9.0 as master",

In other words, "grab this particular version but trick anyone who depends on this module into thinking we pulled the master development branch".

(Note about my workaround: unfortunately, you may need to remove the entire /vendor/geshi directory each time you run composer with my workaround in place to avoid cryptic errors like "Problem 1 - Removal request for geshi/geshi == 9999999-dev".)

In the past, geshi/geshi recommended the use of dev-master(before the 1.0.9 release) but now they too recommend the use of 1.0.9. I Just have one question before i commit this path, how it will affect existing users that have dev-master? Composer will change without errors from dev-master to 1.0.9 ?

This is commited, thanks.
But in my tests it gave me an error in composer, it did not change from dev to 1.0.9. It is possible to fix it by removing the geshifilter and requiring it again, i put a note about this in project page.