remove esprima-six, go back to esprima

`esprima-six` introduces regressions that break the browserifying of modules that used to work before the switch away from `esprima`, e.g.

```
$ browserify -r escodegen
SyntaxError: Line 819: Unexpected string while parsing /Users/maxypoo/src/node_modules/escodegen/escodegen.js
```

```
$ browserify -r detective
SyntaxError: Line 1460: Unexpected string while parsing /Users/maxypoo/src/node_modules/detective/node_modules/escodegen/escodegen.js
```

```
Error: Parsing file /tmp/glsl-parser11424-6356-1jwtxly/node_modules/glsl-parser/lib/index.js: Line 73: Comprehension Error
```

as stated by @thlorenz:

```
< thlorenz> better to have stable ES5 parsing than ES6 features (especially if there is always es6ify)
```

so this PR reverts back to `esprima`