MAINT: Fix testing on Rust 1.12 using cuviper's trick again

As seen in rayon and itertools
diff --git a/.travis.yml b/.travis.yml
index 1bd348f..a656734 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,8 +3,15 @@
 matrix:
   include:
     - rust: 1.12.0
+      before_script:
+        # rand 0.4.2 requires rust 1.15, and rand-0.3.22 requires rand-0.4  :/
+        # manually hacking the lockfile due to the limitations of cargo#2773
+        - cargo generate-lockfile
+        - sed -i -e 's/"rand 0.[34].[0-9]\+/"rand 0.3.20/' Cargo.lock
+        - sed -i -e '/^name = "rand"/,/^$/s/version = "0.3.[0-9]\+"/version = "0.3.20"/' Cargo.lock
       env:
       - ALL=' '
+    - rust: 1.15.0
     - rust: stable
       env:
       - FEATURES='unstable quickcheck'