Don't allow '_' in first place of version code

Fixes a regression of 2cb0ff45. Example:

versionCode project.versionCode_plus

931b0a3087/app/build.gradle (L38)

tests/source-files/com.kunzisoft.testcase/build.gradle

@@ -59,6 +59,15 @@ android {
             // Version code : 2018_04_30
             // Version name : 20180430-underscore
         }
+        underscore_first {
+            applicationIdSuffix = ".underscore_first"
+            versionCode _04_30
+            buildConfigField "boolean", "FULL_VERSION", "true"
+            buildConfigField "boolean", "CLOSED_STORE", "true"
+            // ApplicationId : com.kunzisoft.fdroidtest.applicationidsuffix.underscore_first
+            // Version code : 1
+            // Version name : 1.0
+        }
     }
 }