SemVer for spec_version breaks pre-SemVer clients

[trishankatdatadog]

Description of issue or feature request:

0.12 has adopted SemVer for spec_version. However, this strict checking now prevents 0.12 from loading repositories written with 0.11 and less.

Current behavior:

0.12 fails to load repositories written with 0.11 and less.

Expected behavior:

0.12 should be able to load repositories written with 0.11 (and, optionally, less), so that repositories can migrate from 0.11 to 0.12.

[lukpueh]

Addressed in #950