fix eslint warnings

While working on another PR, I noticed these warnings and thought it would be better to fix separately.

Before:

```
$ npm test

> underscore@1.7.0 test /Users/max/Sites/underscore
> phantomjs test/vendor/runner.js test/index.html?noglobals=true && eslint underscore.js test/*.js test/vendor/runner.js

Took 6057ms to run 1119 tests. 1119 passed, 0 failed.

underscore.js
   886:21  warning  Gratuitous parentheses around expression  no-extra-parens
  1121:14  warning  Gratuitous parentheses around expression  no-extra-parens

test/collections.js
  596:10  warning  ['true'] is better written in dot notation   dot-notation
  597:10  warning  ['false'] is better written in dot notation  dot-notation
  614:10  warning  ['true'] is better written in dot notation   dot-notation
  615:10  warning  ['false'] is better written in dot notation  dot-notation

✖ 6 problems
```

After:

```
$ npm test

> underscore@1.7.0 test /Users/max/Sites/underscore
> phantomjs test/vendor/runner.js test/index.html?noglobals=true && eslint underscore.js test/*.js test/vendor/runner.js

Took 5973ms to run 1119 tests. 1119 passed, 0 failed.

test/collections.js
  596:10  warning  ['true'] is better written in dot notation   dot-notation
  597:10  warning  ['false'] is better written in dot notation  dot-notation
  614:10  warning  ['true'] is better written in dot notation   dot-notation
  615:10  warning  ['false'] is better written in dot notation  dot-notation

✖ 4 problems
```