Update branches for semantic versioning.

[rosiel]

GitHub Issue: Islandora/documentation#1907

What does this Pull Request do?

Update default branch to 1.x.

How should this be tested?

Builds are green.

Interested parties

@seth-shaw-unlv @bseeger

[codecov]

Codecov Report

Merging #28 (f96e0a0) into 1.x (ff723e4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                1.x      #28   +/-   ##
=========================================
  Coverage     89.15%   89.15%           
  Complexity      125      125           
=========================================
  Files             6        6           
  Lines           369      369           
  Branches         61       61           
=========================================
  Hits            329      329           
  Misses           19       19           
  Partials         21       21           

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