Only perform coverage analysis for one compiler version.

.travis.yml

@@ -3,6 +3,7 @@ language: d
 d:
   - dmd-2.070.0
   - dmd-2.069.2
+    env: [COVERAGE=true]
   - dmd-2.068.2
 
 sudo: false

travis.sh

@@ -1,5 +1,11 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
-dub fetch doveralls
-dub test -b unittest-cov
-dub run doveralls
+set -ueo pipefail
+
+if [ ! -z "${COVERAGE:-}" ]; then
+    dub fetch doveralls
+    dub test -b unittest-cov
+    dub run doveralls
+else
+	dub test
+fi