Remove go 1.12 support (goproxy syntax issue)

.travis.yml

@@ -1,14 +1,14 @@
 language: go
+
 go:
-  - "1.12.x"
   - "1.13.x"
-sudo: false
+
 before_install:
   # Make sure travis builds work for forks
   - mkdir -p $TRAVIS_BUILD_DIR $GOPATH/src/github.com/wtfutil
   - test ! -d $GOPATH/src/github.com/wtfutil/wtf && mv $TRAVIS_BUILD_DIR $GOPATH/src/github.com/wtfutil/wtf || true
   - export TRAVIS_BUILD_DIR=$HOME/gopath/src/github.com/wtfutil/wtf
   - cd $HOME/gopath/src/github.com/wtfutil/wtf
-  - export GOPROXY="https://proxy.golang.org,direct" && export GO111MODULE=on
+  - export GOPROXY="https://proxy.golang.org,direct"
 
 script: go get ./... && ./scripts/check-uncommitted-vendor-files.sh && go test -v github.com/wtfutil/wtf/...

go.mod

@@ -1,6 +1,6 @@
 module github.com/wtfutil/wtf
 
-go 1.12
+go 1.13
 
 require (
 	code.cloudfoundry.org/bytefmt v0.0.0-20190819182555-854d396b647c