Ask devs to email chapel_admin with clas and commit access requests.

Instead of mailing chapel-developers@, we are now asking people to send admin
requests (i.e. GitHub team access, CLAs) to chapel_admin@cray.com.
Obfuscate email adderss to hopefully avoid spam.

Based on review feedback from @bradcray.
Merge pull request #19 from thomasvandoren/use-admin-email

Ask devs to email chapel_admin with clas and commit access requests.