Bump version to 2.0.0-pre.

The earlier commit to change the version to 1.3.0 was a mistake. The
major version needs to be bumped due to changes to the database code
which are not backwards compatible.

Author: jholdstock

README.md

@@ -65,7 +65,7 @@ mining, a certificate from an authority (`CA`) like
   command without cloning this repository:
 
   ```sh
-  $ go install github.com/decred/dcrpool@v1.3.0
+  $ go install github.com/decred/dcrpool@release-v2.0.0
   ```
 
   Using `@master` instead will perform a build using the latest code from the

version.go

@@ -40,7 +40,7 @@ var (
 	// the app will panic at runtime.  Of particular note is the pre-release
 	// and build metadata portions MUST only contain characters from
 	// semanticAlphabet.
-	version = "1.3.0-pre"
+	version = "2.0.0-pre"
 
 	// NOTE: The following values are set via init by parsing the above version
 	// string.