Correct boolean inversion and overrided env vars

.travis.yml

@@ -1,20 +1,18 @@
 language: rust
 
-env:
-  - RUSTFLAGS="-D warnings"
-
 matrix:
   fast_finish: true
   include:
     - rust: nightly
       os: linux
-      env: BUILD_DOCS=1 BUILD_BOOK=1
+      env: RUSTFLAGS="-D warnings" BUILD_DOCS=1 BUILD_BOOK=1
     - rust: nightly
       os: osx
       osx_image: xcode9.2
-      env: BUILD_DOCS=1
+      env: RUSTFLAGS="-D warnings" BUILD_DOCS=1
     - rust: nightly-x86_64-pc-windows-msvc
       os: windows
+      env: RUSTFLAGS="-D warnings"
 
 before_script:
   - rustup component add rustfmt