Use Xenial image and fix find call

.travis.yml

@@ -1,12 +1,13 @@
 cache: cargo
 before_cache:
-    - find ./target/debug -type f -maxdepth 1 -delete
+    - find ./target/debug -maxdepth 1 -type f -delete
     - rm -fr ./target/debug/{deps,.fingerprint}/{*ra_*,*test*,*tools*,*gen_lsp*,*thread_worker*}
     - rm -f  ./target/.rustc_info.json
 
 matrix:
     include:
         - os: linux
+          dist: xenial
           language: rust
           rust: stable
           script: