Bump acorn from 5.5.3 to 5.7.4 (vuejs#279)

Author: dependabot[bot]

yarn.lock

@@ -962,9 +962,9 @@ acorn-dynamic-import@^3.0.0:
     acorn "^5.0.0"
 
 acorn@^5.0.0:
-  version "5.5.3"
-  resolved "https://registry.yarnpkg.com/acorn/-/acorn-5.%3Cspan%20class="x x-first x-last">5.3.tgz#f473dd47e0277a08e28e9bec5aeeb04751f0b8c9"
-  integrity sha512-jd5MkIUlbbmb07nXH0DT3y7rDVtkzDi4XZOUVWAer8ajmF/DTSSbl5oNFyDOl/OXA33Bl79+ypHhl2pN20VeOQ==
+  version "5.7.4"
+  resolved "https://registry.yarnpkg.com/acorn/-/acorn-5.%3Cspan%20class="x x-first x-last">7.4.tgz#3e8d8a9947d0599a1796d10225d7432f4a4acf5e"
+  integrity sha512-1D++VG7BhrtvQpNbBzovKNc1FLGGEE/oGe7b9xJm/RFHMBeUaUGpluV9RLjZa47YFdPcDAenEYuq9pQPcMdLJg==
 
 agentkeepalive@^2.2.0:
   version "2.2.0"