Disabled GO111MODULE environment variable on CI systems.

Now that we're using Go 1.12 everywhere, this environment variable is no
longer necessary to enable Go modules. In Go 1.12, it only enables
module commands outside of a module directory, which we don't need.

.travis.yml

@@ -7,12 +7,10 @@ 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

@@ -10,7 +10,6 @@ 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"