Increase the linter deadline from 1 min to 5 min. (#220)

Hopefully will avoid spurious test failures like
https://travis-ci.org/matrix-org/dendrite/builds/272960127

linter.json

 {
     "Vendor": true,
     "Cyclo": 12,
-    "Deadline": "60s",
+    "Deadline": "5m",
     "Enable": [
         "vetshadow",
         "gotype",