Update ruby-csv package to 3.1.7.
## 3.1.7 - 2020-08-04
### Improvements
* Improved document.
[GitHub#158][GitHub#160][GitHub#161]
[Patch by Burdette Lamar]
* Updated required Ruby version to 2.5.0 or later.
[GitHub#159]
[Patch by Gabriel Nagy]
* Removed stringio 0.1.3 or later dependency.
### Thanks
* Burdette Lamar
* Gabriel Nagy
## 3.1.6 - 2020-07-20
### Improvements
* Improved document.
[GitHub#127][GitHub#135][GitHub#136][GitHub#137][GitHub#139][GitHub#140]
[GitHub#141][GitHub#142][GitHub#143][GitHub#145][GitHub#146][GitHub#148]
[GitHub#148][GitHub#151][GitHub#152][GitHub#154][GitHub#155][GitHub#157]
[Patch by Burdette Lamar]
* `CSV.open`: Added support for `undef: :replace`.
[GitHub#129][Patch by Koichi ITO]
* `CSV.open`: Added support for `invalid: :replace`.
[GitHub#129][Patch by Koichi ITO]
* Don't run quotable check for invalid encoding field values.
[GitHub#131][Patch by Koichi ITO]
* Added support for specifying the target indexes and names to
`force_quotes:`.
[GitHub#153][Reported by Aleksandr]
* `CSV.generate`: Changed to use the encoding of the first non-ASCII
field rather than the encoding of ASCII only field.
* Changed to require the stringio gem 0.1.3 or later.
### Thanks
* Burdette Lamar
* Koichi ITO
* Aleksandr
03ce15f09f