Switched Travis and AppVeyor to Go 1.12 for CVE-2019-9634 fix.

This commit brings Travis and AppVeyor up to Go 1.12 to incorporate the
fix for CVE-2019-9634 (which hasn't yet been backported to Go 1.11).
This breaks with our tradition of pinning release branches to a specific
Go release, but it's necessary since this fix won't be backported until
Go 1.11.10:

golang/go#30989

.travis.yml

@@ -42,7 +42,7 @@ addons:
 # than a float to be truncated:
 # https://github.com/travis-ci/travis-ci/issues/9247
 go:
-  - "1.11.9"
+  - "1.12.4"
 
 # Skip the default install step since it doesn't make sense for Mutagen.
 install: true

appveyor.yml

@@ -9,7 +9,7 @@ clone_folder: C:\mutagen
 
 # Set Go versions and environment variables.
 environment:
-  GOROOT: C:\go111
+  GOROOT: C:\go112
   GO111MODULE: "on"
   MINGW_PATH: C:\mingw-w64\x86_64-7.2.0-posix-seh-rt_v5-rev1
   MUTAGEN_TEST_END_TO_END: "full"