Fix parsing issue :

danger#195

Author: afaucogney

Diff for: danger-kotlin-library/src/main/kotlin/systems/danger/kotlin/models/gitlab/GitLab.kt

@@ -38,7 +38,7 @@ data class GitLabMergeRequest(
     @SerialName("allow_maintainer_to_push")
     val allowMaintainerToPush: Boolean = false,
     @SerialName("approvals_before_merge")
-    val approvalsBeforeMerge: Int = 0,
+    val approvalsBeforeMerge: Int? = null,
     val assignee: GitLabUser? = null,
     val author: GitLabUser,
     @SerialName("changes_count")