Build: Change Greenplum main version from 5.0 to 4.3

Jira: PIVOTAL-88

Changed the default version used for the Greenplum main development
branch (and supporting dev branches) to 4.3. Also changed the current
MADlib version to 0.6dev
Build System: Change malformed PGXN version string.

Jira: MADLIB-761

It turned out PGXN version string should be x.y.z{modifier} and we missed
z.  This might be applied to v0.5 branch, but we keep it only in master
for now.