Build features separately.

Compiling rocket and actix-web together presents problems, since ring - a
native dependency - cannot have more than one version linked.

.travis.yml

@@ -7,7 +7,10 @@ before_script:
   - export PATH=$HOME/.cargo/bin:$PATH
 
 script:
-  - cargo test --all --all-features
+  - cargo build --features="iron"
+  - cargo build --features="rocket"
+  - cargo build --features="actix-web"
+  - cargo test --all
   - |
     if command -v cargo-clippy > /dev/null; then
       CLIPPY_STATUS=0

maud/src/lib.rs

@@ -199,7 +199,7 @@ mod actix_support {
     impl Responder for PreEscaped<String> {
         type Item = HttpResponse;
         type Error = Error;
-        fn respond_to<String>(self, _req: &HttpRequest<String>) -> Result<Self::Item, Self::Error> {
+        fn respond_to<S>(self, _req: &HttpRequest<S>) -> Result<Self::Item, Self::Error> {
             Ok(HttpResponse::Ok().body(self.0))
         }
     }