Updated mjml to version 4.6.3 to fix CVE-2020-12827

[Zozman]

According to the National Vulnerability Database at https://nvd.nist.gov/vuln/detail/CVE-2020-12827 , there is a security vulnerability in any version of mjml less than 4.6.3 that "contains a path traversal vulnerability when processing the mj-include directive within an MJML document."

Therefore bumping the version of mjml used by this package to not be susceptible to the vulnerability.