postrelease should not check for towncrier fragments

[mauritsvanrees]

I just released zestreleaser.towncrier 1.0.3. (Ah, should have been 1.1.0...) During postrelease, the new check was needlessly done:

INFO: Starting postrelease.
Found 0 towncrier newsfragments with recognized extension.

WARNING: No towncrier newsfragments found.
The changelog will not contain anything interesting.

Do you want to continue anyway? (y/N)?  y
INFO: Doing dry-run of towncrier to see what would be changed: /Users/maurits/community/zestreleaser.towncrier/bin/towncrier --draft --version t.b.d. --yes
...

We should not run the check during postrelease.
(A warning that some snippets have been left behind could be okay, but should be superfluous.)

[mauritsvanrees]

For good measure, I have created a branch 1.0.x and rereleased 1.0.2 as 1.0.4, to avoid the new features.
I have rereleased master as 1.1.0.

[mauritsvanrees]

Fix released in 1.2.0.