Fix MIT license for pkg.go.dev; tidy go.mod and create go.sum (#62)

Currently the documentation for github.com/kr/pretty is not displayed on
pkg.go.dev because of license restrictions:

    https://pkg.go.dev/github.com/kr/pretty?tab=doc

This is almost certainly because the License file includes an errant
header:

    The MIT License (MIT)

The actual MIT license does not include this header:

    https://opensource.org/licenses/MIT

GitHub and other interfaces surface the type of the license through
automatic detection (as will pkg.go.dev). Hence this header is
superfluous in any case.

Whilst we are at it, tidy up go.mod and add go.sum.
diff --git a/License b/License
index 05c783c..480a328 100644
--- a/License
+++ b/License
@@ -1,5 +1,3 @@
-The MIT License (MIT)
-
 Copyright 2012 Keith Rarick
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
diff --git a/go.mod b/go.mod
index 1e29533..9a27b6e 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
-module "github.com/kr/pretty"
+module github.com/kr/pretty
 
-require "github.com/kr/text" v0.1.0
+go 1.12
+
+require github.com/kr/text v0.1.0
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..714f82a
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,3 @@
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=