Revert "Disabled GO111MODULE environment variable on CI systems."

This reverts commit d5b8540.

It turns out that this variable is somewhat necessary, even with Go
1.12, because if a CI system clones Mutagen into $GOPATH/src (which
happens on Travis), and GO111MODULE is unset or set to "auto" (which is
the default and also explicitly done on Travis), then module support is
disabled. Until GO111MODULE=on is the default, let's continue to set
this, even on AppVeyor (where it may not be necessary since we don't
clone into %GOPATH%/src).

Looking forward, it looks like Go 1.13 may not make GO111MODULE=on the
default after all, but it looks like it may remove the
disable-if-in-$GOPATH/src behavior, so we may be able to get rid of this
setting at that point. See golang/go#31857 for more information.

.travis.yml

@@ -7,10 +7,12 @@ matrix:
     - os: osx
       osx_image: xcode9.4
       env:
+        - GO111MODULE=on
         - MUTAGEN_TEST_END_TO_END=full
         - MUTAGEN_TEST_SSH=true
     - os: linux
       env:
+        - GO111MODULE=on
         - MUTAGEN_TEST_END_TO_END=full
         - MUTAGEN_TEST_SSH=true
         - MUTAGEN_TEST_DOCKER=true

appveyor.yml

@@ -10,6 +10,7 @@ clone_folder: C:\mutagen
 # Set Go versions and environment variables.
 environment:
   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"
   MUTAGEN_TEST_DOCKER: "true"