Fix IE11 'findIndex' error (#89)

* Use custom 'findIndex' function rather than instance method

* install npm@5

* I prefer to up the version, thanks. :-)

.travis.yml

@@ -1,7 +1,7 @@
 language: node_js
 
 before_install:
-  - npm install -g npm@latest
+  - npm install -g npm@5
 
 node_js:
   - "6"
@@ -15,4 +15,4 @@ script:
 
 after_success:
   - npm run test:cov
-  - npm run test:codecov
+  - npm run test:codecov

src/lruCache.js

@@ -1,8 +1,19 @@
+const findIndex = (arr, fn) => {
+  for (let i = 0; i < arr.length; i++) {
+    if (fn(arr[i])) {
+      return i;
+    }
+  }
+
+  return -1;
+}
+
 export default function lruCache(limit, equals) {
   const entries = []
 
   function get(key) {
-    const cacheIndex = entries.findIndex(entry => equals(key, entry.key))
+    entries 
+    const cacheIndex = findIndex(entries, entry => equals(key, entry.key))
 
     // We found a cached entry
     if (cacheIndex > -1) {