Prepend slash to top level dirs in npmignore

[jessebeach]

"Lines prefixed with / will only match entries starting from the current location of that .npmignore. For example, /foo will match foo and not lib/foo, but foo will match both."

from https://github.com/npm/npm/wiki/Files-and-Ignores

I realized that the dirs in .npmignore should be prepended with / to indicate that they are top-level.

I noticed that the src subdir in __tests__ was missing in the package. This change brings those back. npm pack produces:

__mocks__/
__tests__/
.babelrc
.eslintignore
.eslintrc
.npmignore
CHANGELOG.md
docs/
lib/
LICENSE.md
package.json
README.md
scripts/

[coveralls]

Coverage remained the same at 99.57% when pulling 04a41b1 on jessebeach:fix-npmignore into 71ce135 on evcohen:master.

[beefancohen]

Good catch!

[beefancohen]

v6.0.2 published!