Update source installation requirements (#3124)

* Update source installation requirements

We require Golang 1.8 since we use `net/url.PathEscape` which was not introduced until then

* Use Golang 1.8 for build and test

If we state Golang 1.8 we should test using that version as well

* Use golang 1.8 only for single build to see if everything compiles

* Change back edge also for commented out sqlite test

.drone.yml

@@ -56,7 +56,7 @@ pipeline:
       event: [ push, tag, pull_request ]
 
   build-without-gcc:
-    image: webhippie/golang:edge
+    image: webhippie/golang:1.8
     pull: true
     environment:
       GOPATH: /srv/app

docs/content/doc/installation/from-source.en-us.md

@@ -17,7 +17,7 @@ menu:
 
 This section will not include basic [installation instructions](https://golang.org/doc/install).
 
-**Note**: Go version 1.7 or higher is required
+**Note**: Go version 1.8 or higher is required
 
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