The .env.sample file contained a few comments using `//`. The problem is
that `//` is not supported by vlucas/phpdotenv and this was resulting in
the following fatal error when following the dev environment setup
instructions and running `./do install`:
```
ERROR: Uncaught Dotenv\Exception\InvalidFileException: Failed to parse dotenv file due to unexpected whitespace. Failed at ["" // URL of your site (used for multisite env and equals to DOMAIN_CURRENT_SITE from wp-config.php)]. in /srv/www/mailpoet/public_html/wp-content/plugins/mailpoet/vendor/vlucas/phpdotenv/src/Parser.php:148
```
To fix this problem, this commit replaces all instances of `//` in
.env.sample with the `#` (see
https://github.com/vlucas/phpdotenv#comments).
Quick test show that `//` doesn't work and `#` does:
```
>>> Dotenv\Dotenv::parse("FOO=Bar // some comment")
Dotenv\Exception\InvalidFileException with message 'Failed to parse dotenv file. Encountered unexpected whitespace at [Bar // some comment].'
>>> Dotenv\Dotenv::parse("FOO=Bar # some comment")
=> [
"FOO" => "Bar",
]
```
