increase placeholder slot length to 255 chars.

fixes #3359
slot can now have up to 255 characters
Merge pull request #3362 from czpython/issue-3359

increase placeholder slot length to 255 chars.
Add django 1.7 migration for modified slot length
Fixes #3364

Removed links to crate.io
Merge pull request #3365 from maestrofjp/remove-crate-io-links

Removed links to now defunct crate.io
Merge pull request #3366 from yakky/feature/fix_1_7_migration

Add django 1.7 migration for modified slot length