libmatekeyring-1.5.1: Version bump + misc fixes

Bump version to 1.5.1

Now fully autogenerated automagic so:
* Remove file configure from sed
* Move mate_src_prepare after sed.

Also configure.in was renamed to configure.ac [1]

[1] https://github.com/mate-desktop/libmatekeyring/commit/62493ad2b8d3c1f9a544b783629333b961bd4ea1