Speed up travis by using release mode

before we built in debug mode for testing,
but then installed miri, which builds in release mode.

So we built in release mode anyway but tested slowly in debug mode

.travis.yml

@@ -15,8 +15,8 @@ script:
   xargo/build.sh
 - |
   # Test plain miri
-  cargo build &&
-  cargo test &&
+  cargo build --release &&
+  cargo test --release &&
   cargo install
 - |
   # Test cargo miri
@@ -26,11 +26,11 @@ script:
   cd ..
 - |
   # and run all tests with full mir
-  MIRI_SYSROOT=~/.xargo/HOST cargo test
+  MIRI_SYSROOT=~/.xargo/HOST cargo test --release
 - |
   # test that the rustc_tests binary compiles
   cd rustc_tests &&
-  cargo build &&
+  cargo build --release &&
   cd ..
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