Make postcompile.sh script work on macOS

[lpinca]

Note: the engine.io.js file is the generated output of make engine.io.js, and should not be manually modified.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

The npm run compile command does not work on macOS due to an invalid usage of sed in the postcompile.sh script

New behaviour

npm run compile runs without errors on macOS.

Other information (e.g. related issues)

First commit

build: use env in shebang

Use the `bash` executable that appears first in user's `PATH`.

Second commit

build: make postcompile.sh script work on macOS
    
On macOS the script is currently interpreted as the value of the `-i`
option. Use an empty extension for the `-i` option and the `-e` option
to specify the script.

[lpinca]

It looks like it does not work on Linux.

Fixed.

[darrachequesne]

@lpinca thanks 👍