Use comma separated features in cargo invocation
diff --git a/.travis.yml b/.travis.yml
index 6c92a16..3632176 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -32,18 +32,18 @@
           cargo build --no-default-features &&
           cargo build &&
           cargo build --features full &&
-          cargo build --features 'visit fold' &&
-          cargo build --features 'full visit fold' &&
-          cargo build --no-default-features --features 'full parsing printing' &&
-          cargo build --no-default-features --features 'visit fold parsing printing' &&
-          cargo build --no-default-features --features 'full visit fold parsing printing'
+          cargo build --features fold,visit,visit_mut &&
+          cargo build --features full,fold,visit,visit_mut &&
+          cargo build --no-default-features --features full,parsing,printing &&
+          cargo build --no-default-features --features fold,visit,visit_mut,parsing,printing &&
+          cargo build --no-default-features --features full,fold,visit,visit_mut,parsing,printing
           ;;
         test)
           cargo test --all-features --release
           ;;
         clippy)
           cargo install clippy --debug --force || exit
-          cargo clippy --features 'full visit fold' -- -Dclippy
+          cargo clippy --features full,fold,visit,visit_mut -- -Dclippy
           ;;
         codegen)
           (cd syn_codegen && cargo run) &&
@@ -66,7 +66,7 @@
         curl -sSLo synom.crate.tgz "https://crates.io/api/v1/crates/synom/${SYNOM}/download" &&
         tar xf synom.crate.tgz &&
         mv "synom-${SYNOM}" synom &&
-        cargo doc --features 'full visit fold' &&
+        cargo doc --features full,fold,visit,visit_mut &&
         travis-cargo doc-upload
       fi