main.coffee: Fix old version warning

This fixes the old version warning by properly compare
the version numbers between `result` and `COALA_MIN_VERSION`
instead of comparing these strings lexicographically.

Closes coala#48

lib/main.coffee

@@ -2,6 +2,29 @@
 helpers = require 'atom-linter'
 path = require 'path'
 
+# Test if the version number v1 is older than v2
+isOlderThan = (v1, v2) ->
+    subversions1 = v1.split('.')
+    subversions2 = v2.split('.')
+    for idx in [0..3]
+        unless Number(subversions1[idx] == Number(subversions2[idx]))
+            return Number(subversions1[idx]) < Number(subversions2[idx])
+    # If the three first version numbers were equal.
+    if isDevVersion(v1) && isDevVersion(v2)
+        return devNumber(v1) < devNumber(v2)
+    else if isDevVersion(v1) && !(isDevVersion(v2))
+        return true
+    # If the version numbers are equal, then v1 is not older than v2.
+    return false
+
+isDevVersion = (v) ->
+    subversions = v.split('.')
+    return (subversions.length > 3) && (subversions[3].slice(0, 3) == 'dev')
+
+devNumber = (v) ->
+    subversions = v.split('.')
+    return Number(subversions[3].slice(3, subversions.length))
+
 COALA_MIN_VERSION = '0.10.0'
 module.exports =
   config:
@@ -29,7 +52,7 @@ module.exports =
 
     # Check version of coala
     version = helpers.exec(@executable, ['--version']).then (result) ->
-      if result <= COALA_MIN_VERSION
+      if isOlderThan(result, COALA_MIN_VERSION)
         atom.notifications.addError \
           'You are using an old version of coala !',
           'detail': 'Please upgrade your version of coala.\n