Upgrade to acorn 6

[ctavan]

Currently a very old version of acorn-jsx is being used to parse occurences of the <Trans> component.

This leads to lots of errors when people use modern JavaScript syntax that cannot be parsed by the ancient acorn.

This patch upgrades to the latest acorn-jsx version and comes with a custom version of acorn-jsx-walk based on acorn-walk.

Should fix:

• Extraction breaks for whole file when using shorthand JSX fragments #86
• <Trans> component in Typescript files is no longer parsed #88
• React class parse error #93

[coveralls]

Coverage increased (+0.6%) to 91.463% when pulling 0424c5e on contentpass:upgrade-to-acorn-6 into 60be465 on i18next:master.

[coveralls]

Coverage decreased (-1.3%) to 89.634% when pulling 58a5df2 on contentpass:upgrade-to-acorn-6 into 60be465 on i18next:master.

[cheton]

@ctavan This definitely helps address these issues. Thank you.