fix ci (#93)

.drone.yml

@@ -10,8 +10,6 @@ steps:
 - name: cargo build
   image: rustlang/rust:nightly
   volumes:
-  - name: cache
-    path: /usr/local/cargo
   - name: target-cache
     path: /drone/src/target
   commands:
@@ -19,17 +17,12 @@ steps:
 - name: cargo test
   image: rustlang/rust:nightly
   volumes:
-  - name: cache
-    path: /usr/local/cargo
   - name: target-cache
     path: /drone/src/target
   commands:
   - cargo test
 
 volumes:
-- name: cache
-  host:
-    path: /home/drone/.cargo
 - name: target-cache
   host:
     path: /home/drone/cargo-cache