Update dates in manpages to match the NEWS entry

doc/man/kdig.1

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-.TH "KDIG" "1" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KDIG" "1" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 kdig \- Advanced DNS lookup utility
 .

doc/man/keymgr.8

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-.TH "KEYMGR" "8" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KEYMGR" "8" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 keymgr \-  DNSSEC key management utility
 .

doc/man/khost.1

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-.TH "KHOST" "1" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KHOST" "1" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 khost \- Simple DNS lookup utility
 .

doc/man/knot.conf.5

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-.TH "KNOT.CONF" "5" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNOT.CONF" "5" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knot.conf \- Knot DNS configuration file
 .

doc/man/knot1to2.1

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-.TH "KNOT1TO2" "1" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNOT1TO2" "1" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knot1to2 \- Knot DNS configuration conversion utility
 .

doc/man/knotc.8

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-.TH "KNOTC" "8" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNOTC" "8" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knotc \- Knot DNS control utility
 .

doc/man/knotd.8

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-.TH "KNOTD" "8" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNOTD" "8" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knotd \- Knot DNS server daemon
 .

doc/man/knsec3hash.1

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-.TH "KNSEC3HASH" "1" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNSEC3HASH" "1" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knsec3hash \- Simple utility to compute NSEC3 hash
 .

doc/man/knsupdate.1

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-.TH "KNSUPDATE" "1" "2015-04-28" "2.0.0-beta" "Knot DNS"
+.TH "KNSUPDATE" "1" "2015-04-23" "2.0.0-beta" "Knot DNS"
 .SH NAME
 knsupdate \- Dynamic DNS update utility
 .