Enable encryption by default

[allipatev]

Since DB version 8 encryption is enabled by default (see Release Notes - Database).

However, r-exasol by default establishes a non-encrypted connection, so the following kind of connection

exaconn <- dbConnect(
    drv = "exa",
    exahost = "demodb.exasol.com:8563",
    uid = "<YOUR_USER>",
    pwd = "<YOUR_PASSWORD>")

fails with

Error in odbcGetInfo(con) : argument is not an open RODBC channel
In addition: Warning messages:
1: In odbcDriverConnect(con_str) :
  [RODBC] ERROR: state 28000, code -3732480, message [Exasol][Exasol Driver]Connection exception - Only TLS connections are allowed.
2: In odbcDriverConnect(con_str) : ODBC connection failed

Please align r-exasol with the current DB approach and make encryption enabled by default.

[tomuben]

It's possible that this is the same issue as #153