Fixed #550: There was opportunity for nil pointers in lock

cfg/lock.go

diff --git a/cfg/lock.go b/cfg/lock.go
index 62d08ef..ffba610 100644
--- a/cfg/lock.go
+++ b/cfg/lock.go
@@ -218,7 +218,7 @@
 			}
 		}
 		if !found {
-			lf.DevImports[i] = LockFromDependency(tds[i])
+			lf.DevImports = append(lf.DevImports, LockFromDependency(tds[i]))
 		}
 	}